In a remarkable musical diversion, two of pop's biggest stars are treading the opera stage in Paris. They tell Neil McCormick why.

Sting sits in a cross-legged lotus position on a sofa, one arm raised, as he stretches his body. "Thanks for the boot in the back last night," he says. "That was a nice touch."

"You like the way I ground my heel in?" retorts Elvis Costello.

"When I'm lying there in fear and pain, I'm not acting," says Sting.

"It's in my contract," says Elvis. "I get to rough you up every night."

Sting and Costello have been rehearsing an opera. Veterans of the Seventies punk and new-wave movement, the two British stars have since established themselves among the most acclaimed and wide-ranging singer-songwriters in the world. But their new project promises to be something of a departure. From tomorrow, for five nights at the historic Théatre Du Chatelet in Paris, they make their debuts as operatic singers in the premiere of the extraordinary Welcome to the Voice.

"It is important sometimes to get outside your comfort zone just to see what happens," says Sting. "Neither of us has done this out of a sense that we're great enough to do opera now. We are here to learn something, but I think that about most things. We are here to learn about how we can expand what we do as rock singers."

"The biggest difference is running around the bloody stage doing stuff while trying to remember to sing the music," says Costello.

"In a frock," adds Sting.

"In a voluminous coat," Costello corrects him, referring to the huge black garment in which his character is perpetually swathed. "But that's OK. The coat is doing a lot of the acting."

The opera was composed by Costello's long-serving keyboard player, Steve Nieve, with a libretto by Nieve's wife, French writer and psychoanalyst Muriel Teodori. Sting plays the male lead, Dionysos, a working man whose love of opera and attraction to a diva (played by rising opera star Sylvia Schwartz) threatens the cultural status quo, as enforced by Costello's fascistic police chief of Police. Both lent their voices to last year's studio recording of the work, released by Deutsche Grammophon.

It is a compelling piece that mixes popular and operatic singing, embracing full orchestral arrangements, some electronic flavour and jazz spontaneity. Originally workshopped at a New York jazz festival in 2000, it has slowly made its way to the stage and is now a lavish, extravagantly designed production, directed by Wolfgang Doerner with the Ensemble Orchestral de Paris.

"I am still a little incredulous to find myself here," says Sting. "If someone had asked me directly to star in an opera, my response would have been 'There is no way you are getting me in white tights'. I think we are required to bring some of the chaotic or spontaneous energy of rock to the piece, rather than stand there like operatic tenors and just sing. We are being asked to be ourselves in a way, volatile elements in this very precious form, interlopers in the opera."

"Actually it's as much a jump for the opera singers as it is for us," says Costello. More of an opera buff than Sting (with an unfinished opera of his own, The Secret Arias, workshopped as an oratorio by the Royal Danish Opera in 2005), Costello certainly understands the discipline. "Because it's a modern, untried piece, they are not on certain ground. They know it's a one-of-a-kind experience. They're good humoured about it - they've probably worked out that we're not going to turn up in their next production."

It is instructive to watch the different ways Costello and Sting occupy a room. Costello looks like an intellectual gangster, hunched in an armchair, stocky frame pressing tight against his jacket, pork-pie hat on his head, blue-lensed glasses hovering over a scruffy, red-tinged beard. He manages to convey an air of slovenly relaxation and tightly coiled energy at the same time. Naturally loquacious, he dominates the conversation. In commercial terms he comes nowhere near Sting, but as something of a musical polymath his senior status seems to go unchallenged.

Sting meanwhile seems graciously content to let his colleague run the conversation. In black trousers and tight sweater, with a thick, greying beard, he sits listening in a yoga position, occasionally interjecting in a soft voice. When Sting describes opera as "a fantastic art form that is more or less ignored by popular media because it is so elitist and so terrifyingly hermetically sealed," Costello argues to the contrary: "To some degree it's the other way around. Popular media has become very judgmental about opera. No one is actually barring your way from listening. Some rock and roll gigs are as expensive as the most expensive tickets in the opera house, so the idea it's elitist is absurd. Opera recordings are not more expensive than hip hop recordings. It's more about whether people are tuned into that kind of vocal production."

And he then digresses into an explanation of how operatic singing was developed to project into huge theatres, and how microphones changed everything. "There's a famous recording of [Al] Jolson and [Bing] Crosby singing together, and it's one of the most instructive things about the progress of singing in the 20th century because Jolson's still trying to hit the back wall, so he just sounds mad, while Bing is completely convincing, its like he's whispering in your ear."

"There's something about the proximity of the microphone when it's this close that is very sexual," suggests Sting. "You can hear the mouth working, and it's wet. Opera singers are too far away to really get intimate with."

"You can say that, but when Sylvia is singing and we are both called upon to react, it doesn't take a lot of acting," says Costello.

"She sings notes and I get levitated," Sting agrees.

"It's pretty affecting," says Costello. "Then again, we do have the best seats in the house."

The combination of curiosity, adventure and love of music that is driving these two stars to risk this experiment with opera is genuinely heartening. Fresh from one of the highest-grossing concert tours ever with the notoriously combative Police - it took 8 million - Sting seems to be enjoying the dynamics of an ensemble production. "They are very supportive," he says. "You come off stage and they are all hugging you."

Something to Sting about... Surprise. That's what Sting tries to do each time he performs. And that's what he hopes the audience will feel when he takes the stage at the Esplanade Concert Hall next month...

Police fans, get ready to relive all the sights and sounds of The Police world tour. ''The Police: Certifiable'', DVD featuring the band's entire performance from one especially hot night of music in Buenos Aires, Argentina is out now...

BBC News Correspondent Nick Higham caught up with Sting and Edin Karamazov when they were recently in London rehearsing 'Songs from the Labyrinth' as they prepare to perform in Australia and the Far East later this month. Watch BBC World News on Friday, 7 November and BBC Breakfast in the UK (BBC1) on Saturday, 8 November. For an unique insight into playing the lute, watch a tutorial with Sting online at http://news.bbc.co.uk/2/hi/entertainment/7714354.stm,
available from Friday, 7 November...

The singer Sting has made a rare return to his home city to tell its story on canvas rather than in music or lyrics. The former frontman of the Police commissioned one of America's most sought-after artists to paint a portrait of Newcastle...